Method of electropolishing



Patented Apr. 17, 1951 2,549,737 METHOD OF ELECTROPOLISHING Harold J. Wiesner, Elkhart, Ind.,

assignor to C. G.

Conn, Ltd., Elkhart, Ind., a corporation of Indiana No Drawing. Application July 19, 1946, Serial No. 684,796

3 Claims.

This invention relates to a method of and a bath for electropolishing and more particularly to the electropolishing of copper and copper alloys.

One of the objects of the invention is to pro- 'vide a method of and bath for electropolishing in which all of the exposed surfaces of an article to be polished will be given a uniform bright finish free from discoloration.

Another object is to provide a method of and bath for electropolishing copper alloys containing lead which will produce a high luster free from smut.

The bath of this invention consists essentially of ,polyphosphates of the alkali metals and either borates or carbonates mixed therewith in an aqueous solution. The general formula covering this class of phosphates is where M is a monovalent alkali metal and n is an integer equal to or greater than 3. The most satisfactory polyphosphates are those often referred to as the triphosphates or tripolyphosphates, the general formula being M5P301o.

It is to be understood that triphosphate and tripolyphosphate may be used interchangeably throughout this application. Sodium tripolyphosphate, for example, either as the pentahydrate or anhydrous salt may be used, or the corresponding potassium salts. These salts function in solution to form soluble compounds with the copper, zinc and lead and other metals commonly found in the copper base alloys during electropolishing. In the operation of the bath,

the copper plates out as a sludge at the cathode after building up to a concentration of .05-.15 oz./gal. Thus, the net loss of tripolyphosphate is that which combines with the zinc, lead and other metals and that which is lost by drag-out. I have found that a satisfactory concentration is of the order of 12-16 oz. of a tripolyphosphate salt per gallon of solution although this concennation" is not critical, and the bath will function satisfactorily with a concentration of trrpolyphosphate as low a 8 ounces per gallon or up to its saturation point.

The'efiect of the borates and carbonates in the bath is to broaden the current density range and to obtain uniformity of polishing. If these materials are omitted there will be substantially less brightening in the recesses of the work and other low current density areas and etching may occur. Also, discolorations may occur in various areas of the work pieces. Among the specific compounds which I have found to be most satisfactory are boric acid, sodium tetraborate, potassium tetraborate, ammonium carbonate, sodium bicarbonate and potassium bicarbonate which are preferably used in concentrations of from 2 to 12 ounces per gallon. If ammonium carbonate is used, it is necessary to keep the temperature of the bath below about F. as above this temperature it decomposes rapidly. Ammonium carbonate and sodium and potassium bicarbonate also react with acid which may be added to the bath to lower its pH to give oil some carbon dioxide. Boric acid and sodium or potassium tetraborate are preferred since they are thermally stable, do not decompose when acid is added and function to bufier the solution. While the ammonium radical is not in reality a metal, it will be identified hereinafter with sodium and potassium as the ion of an alkali metal.

In addition to these basic constituents of the bath, there may be added either an acid or a base in quantities sufficient to maintain the pH of the bath in the range from 7.0-8.5. For this purpose, sodium, potassium or ammonium hydroxide may be used to raise the pH. The sodium and potassium hydroxides are to be preferred as excessive amounts. of ammonium salts are detrimental. phosphoric, metaphosphoric or orthophosphoric may be used.

If the pH is substantially below 7.0, the triphosphate hydrolyzes rapidly and the life of the bath is appreciably shortened. If on the other hand, the pH is substantially above about 8.5, the brightness of the electropolished surface diminishes.

The current density range may be further extended and greatly increased brightness produced by the addition of orthophosphate salts to the bath. Examples of satisfactory salts are sodium, ammonium or potassium salts such as mono, di or tri sodium or potassium phosphate and mono or di ammonium phosphate. The di sodium, di potassium and di ammonium phosphates, when put into solution, have pH values nearer to those in the operating range of the bath and for this reason are preferred. These salts when added to the bath in concentrations such that the'POt content is about 2.6-6.5 cal/gal. extend the current density range and greatly: increase the brightness of the electropolished surface... Th e above mentioned concentrations correspond for example to about 10 to 24. oz;/gal.. of hydrated di sodium phosphate (NZLZHPOQ .12H2O or about 4.0 to 9.5 oz./gal. of anhydrous di sodium phos phate. This pronounced eliect of the orthophosphate salts is particularlynotice'able in" the case of deeply recessed articles that are electropolished.

In addition to the above mentioned functions, the salts of orthophosphoric acid also serve to buiier the solution. While their effect in broadening the current density and improving the brightness is not noted until a concentrationof For lowering the pH, tripolyabout 4 oz./gal, of the anhydrous salt is attains there will lie :an appreciable buffering "effect well below this-concentration. The 'sa'lts may then be conveniently added at lower concentrations it it is desired only to obtain a more highly buifered solution.

It has further been found that the addition of oxalic acid or one of its *sa-lts su'cl'i ns-sodium oxalate, ammonium oxalate or potassium-oxalate in concentrations of about 4 to 6 ounces per gallon enables the temperature of the :bath to :be increased up to as much as 200 F. with good :results. Without the addition of these materials, it has been found that the surface tends tobecome irregular at temperatures substantially in 'excessof 1 50 F.

In general, I'have 'found that agitation of the work or of the solution during electropolishing is desirable. As a result of using agitation, higher current densities are permitted with less chance of burning the protruding sections of a complicated piece of work. The commonly employed methods of agitation used in the electroplating industry, such as oscillation of the work rod, air agitation, or solutionagitation by means of pumps or paddles are satisfactory. The use of work rod oscillation is perhaps to be preferred.

The time required to polish to a high luster will vary with the particular material being polished and with the operating conditions. In general the time will vary from two to twenty minutes and will average about five to ten minutes.

In polishing according to the present inventionth'e piece to be polished is connected as an anode in 'a bath constituted as described above and current is passed from the anode to a cathode which .is preferably copper or plated with copper. The current density is preferably in i excess of 100 amperes per square foot and may in some cases exceed 150 amperes per square foot. When so operated the pieces will be'brought quickly to a uniform high luster without discoloration or smut.

As examples of-baths which I have foundsatisfactory, the following may be given:

Temperature, "F Cln'rent density (average) 'EXAALPLE-#3 Sodium tripolyphosphate- 8-satu1'ation 14-:l6.oz./gal.

(anhydrous). Boric acid 2-12 oz./gal 4-5 ozjgal. Disodium hydrogen phos- 4-9.5 oz./gal. 6.0-7.0.

EHphate (anhydrous). 'reziifififillFrIILIIIII Current density (average) V Agave 100 amps ;/sq.

Range Breferred EXAMPLE #4 5 .Sn'dium 'trlpo jyphosphate Ssaturation... 14-16 oz./ga1.

(anhydrous). :Boric acid :2-12 oz./gal 4-5 oz./gal. "S'odum oxalate :or oxalic '1-8 oz./gal 4-6 ozJgal.

acid. pH A- 7.0-8.5 7.0-7.5. Temperature, F ice-212 lac-200 F.

.Gurrent.density Aliiive 150 amps/sq.

While the invention has been particularly described in connection with several specific bath compositions, it will be understood that various combinations and concentrations-of the constituents mentioned as Well as various operating conditions might be employed within the scope 'of the invention. The examples given are to be taken as illustrative only, reference being .-had to the appended claims for a determination of the scope of the invention.

What is claimed is:

1. The method of polishing copper and alloys in which copper is a major constituent which comprises connecting the alloy to be polished .as an anode in an aqueous bath consisting essentially of a solution of 'a tripolyphosphate of an alkali metal and a material selected from the group consisting of boric acid, boric acid salts and salts of carbonic :acid, said bath having .a pH in the range from '7;0-8.5.

2. The method of polishing alloys in which copper is a major constituent which comprises connecting the alloy to be polished as an anode in an aqueous bath consisting essentially of a solution of a tripolyphosphate of an alkali metal and a material selected from the group consisting of boric acid, 'boric acid salts and salts of rearbonic acid, maintaining the pH of the bath in the range from 7.0-8.5 and operating the bath at a temperature between 100 F. and 212 'F. and at a current density in excess of 100 amperes per square foot.

3. The method of polishing alloys in which copper is a major constituent which comprises connecting the alloy to be'polished as an anode in an aqueous bath consisting essentially of .a solution of a tripolyphosphate of an alkali metal, a material selected from the group consisting of 'boric acid, boric acid salts and salts of carbonic acid and an-ortho-phosphate salt and maintaining the pH of the bath in the'range from 7;0--8.5.

HAROLD J.

REFERENCES CITED The following references :are :of record in the file of this patent:

UNITED STATES PATENTS Number Name .Date

2,086,867 Hall a--- July 13, 1-937 2,226,384 Norris Dec. .14, 1940 2,244,158 Hubbardetal June .3, .1941 2,294,227 Delaplaee-et a1. Aug. 25, 1942 2,330,170 .Manuel Sept. 21, 194-3 2,428,141 'Burkhardt "Sept. 30, .1 94? '70 FOREIGN PATENTS Number Country .Date

435,710. GreatBritain Sept. 26, 1935 

1. THE METHOD OF POLISHING COPPER AND ALLOYS IN WHICH COPPER IS A MAJOR CONSTITUENT WHICH COMPRISES CONNECTING THE ALLOY TO BE POLISHED AS AN ANODE IN AN AQUEOUS BATH CONSISTING ESSENTIALLY OF A SOLUTION OF A TRIPOLYPHOSPHATE OF AN ALKALI METAL AND A MATERIAL SELECTED FROM THE GROUP CONSISTING OF BORIC ACID, BORIC ACID SALTS AND SALTS OF CARBONIC ACID, SAID BATH HAVING A PH IN THE RANGE FROM 7.0-8.5. 